How I built a Lexware Office integration for Medusa v2

ter recovers from Lexware's 2 req/sec limit and transient 503 errors without workflow interruption.

• Zero-Downtime Configuration: Admin UI-driven toggles eliminate code deployments for tax rate or payment term adjustments.

Core Solution

LexBridge is implemented as a native Medusa v2 module, leveraging TypeScript end-to-end for type safety and maintainability. The architecture decouples business logic from external API constraints through three primary layers:

1. Event-Driven Workflow Subscribers listen to the order.placed event. Upon trigger, the custom service executes:

• Customer resolution: Queries Lexware via VAT ID or email; creates a new contact if unmatched.
• Invoice assembly: Maps line items, applies dynamic tax rates, and injects payment terms.
• PDF retrieval: Fetches the finalized invoice from Lexware and stores the reference ID in the Medusa database.

2. Tax & Payment Term Engine

• Tax Rate Override: Germany enforces split VAT (7% for flowers/food, 19% standard). The plugin exposes a taxRateOverride callback, allowing merchants to define category-based or SKU-based tax logic without modifying core code.
• Payment Provider Mapping: Terms are configured per payment method. PayPal defaults to "due immediately", while B2B invoice payments route to "14 days" net terms. Configuration is stored in Medusa's settings table and resolved at runtime.

3. API Reliability & Security

• Rate Limit Handling: Lexware's 2 req/sec threshold and frequent 503 responses are mitigated via an exponential backoff retry mechanism with jitter. Failed requests queue and resume without blocking the main event loop.
• Idempotency: Medusa v2 may emit duplicate order.placed events during retries or webhook replays. A unique database constraint on order_id guarantees exactly-once invoice generation.
• Credential Management: Lexware API keys are encrypted at rest using AES-256-GCM and decrypted only during active API sessions. SMTP delivery for invoice notifications is handled via Nodemailer, configured through the Admin UI.

Installation & Deployment

npm install medusa-lexbridge

Post-installation, all configuration (API credentials, tax callbacks, payment mappings, SMTP settings) is managed through Medusa's native Admin UI components. No code changes are required after deployment.

Pitfall Guide

1. Hardcoding German VAT Rates: Germany mandates split rates (7% vs 19%). Applying a single rate causes compliance violations and accounting mismatches. Best Practice: Implement the taxRateOverride callback to dynamically resolve rates per line item based on product metadata or category tags.
2. Uniform Payment Term Assignment: Treating all payment methods identically breaks B2B cash flow expectations. Best Practice: Map payment providers to specific Lexware terms via configuration tables rather than conditional logic, ensuring easy updates as payment gateways change.
3. Ignoring Lexware API Throttling: The 2 req/sec limit and aggressive 503 responses will crash synchronous or naive retry implementations. Best Practice: Use exponential backoff with randomized jitter, cap maximum retries, and implement circuit-breaker logic to prevent cascade failures during Lexware outages.
4. Event Duplication & Missing Idempotency: Medusa v2's event system can fire order.placed multiple times. Without safeguards, this generates duplicate invoices and billing disputes. Best Practice: Enforce a unique constraint on order_id in the plugin's database schema and verify invoice existence before initiating API calls.
5. Plaintext API Key Storage: Storing Lexware credentials in .env files or unencrypted database columns exposes them to breaches and violates GDPR/security best practices. Best Practice: Encrypt keys using AES-256-GCM at rest, rotate them periodically, and restrict decryption to the service layer only during active API transactions.
6. Bypassing Admin UI for Business Logic: Embedding tax rules or payment mappings directly in service code requires deployments for minor changes. Best Practice: Expose all configuration toggles through Medusa's native Admin UI components, enabling merchants to adjust workflows without touching the codebase.

Deliverables

• Architecture Blueprint: Complete Medusa v2 → LexBridge → Lexware Office data flow diagram, including event subscriber chains, database schema constraints, and API request lifecycle.
• Deployment & Validation Checklist: Pre-flight requirements (API credentials, tax mapping verification, payment provider alignment), installation steps, environment variable encryption setup, and post-install sanity tests.
• Configuration Templates: Ready-to-use JSON/YAML snippets for taxRateOverride logic, payment term mappings, SMTP/Nodemailer settings, and Admin UI component registration.